When you first created your SharePoint list form, did you select the option "Manage multiple list items with this form"? If you did then I think that's the reason why you're getting such behavior.
Essentially, each field that you add in a SharePoint list form will be added as a column in the SharePoint list associated with it. So if you want to be able to manage (submit new items and/or update existing ones), multiple list items with your form, the entire data source needs to be repeating. In this case, each row from the repeating group corresponds to one item in your SharePoint list; whereas if you did not select the 'manage multiple items' option your data source will not be repeating and therefore each form will correspond to one list item.
I hope that makes sense, feel free to respond back if you need more information.